Antenna tuning unit



Oct. 7, 1958 A. G. KANDOIAN 2,855,599

ANTENNA TUNING UNIT Filed Aug. 5,1955

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4 l 4 we ANTENNA w/mour ANTEW-A/A wmv ram/v4 (O/L GOA INVENTOR IM-ARM/6' QAANQO/A/V l I I 2M6 QMC f ATTORNEY United States Patent OANTENNA TUNING UNIT Armig G. Kandoian, Glen Rock, N. J., assignor toInternational Telephone and Telegraph Corporation, Nutley, N J., acorporation of Maryland Application August 5, 1955, Serial No. 526,716

6 Claims. (Cl. 343-861) This invention relates to antenna tuning unitsand, more particularly, to an antenna tuning unit whichsimultaneouslymatches the input impedance from a transmission line to anantenna to within a predetermined standing wave ratio over a widefrequency range while tuning the antenna.

When a transmitter operates into a transmission line having a givencharacteristic impedance such as 50 ohms, the combined effect of thedistributed capacitance and inductance of the transmission line is topresent a pure 50 ohm resistance (characteristic impedance) at the inputof the line, which accepts power like an actual resistor but, instead ofdissipating the accepted power as heat, causes the energy from thetransmitter to travel down the transmission line in the form of a guidedelectromagnetic wave, called the incident wave. In the simplified case,this incident wave is the only efiect present and the measured inputimpedance of the transmission line is simply 50 ohms, or thecharacteristic impedance. If the transmission line extends to infinitelength or terminates in its characteristic impedance, simulating aninfinite length, the load at the output terminals of the transmissionline completely absorbs the energy of the incident wave as it arrivesand thus, if the load is an antenna, maximum power is radiated. However,if the transmission line terminates in a load having an impedancediffering from the characteristic impedance of the line, some energy isreturned from the load, back into the transmission line in the form of areflected electromagnetic wave traveling toward the input terminal. Atthe input terminal, this reflected wave appears asa voltage in serieswith the characteristic impedance.- Depending upon the phaserelationships, this reflected voltage may either aid or oppose the inputvoltage applied to the input terminals and thus, increase or decreasethe input current. The effect in input impedance is higher if thereflected voltage opposes the flow of current and lower if the length ofline is such that the reflected voltage arrives in aiding phase to theinput voltage.

In every case where the line is not matched to the load or, in otherwords, where the load does not equal the characteristic impedance of theline, there is a reflected and incident wave present on the transmissionline and 2,855,599 Patented Oct. 7, 1958 a reflected wave, or a SWRgreater than 1, indicates that some of the input power is not beingabsorbed or utilized by the load. In an extreme case, such as a shortcircuit.

voltage becomes zero giving an infinite SWR. Obviously,

to avoid excessive voltage stress in the transmission line and, equallyimportant, to deliver maximum useful power to the load, it is necessaryto minimize the reflected wave with respect to the incident power and toadjust the load for the lowest standing wave ratio. i a

It has long been known that a typical antennadesigried for optimumoperation at a given frequency may beus'ed for transmission over a rangeof frequencies provided the antenna is tuned and matched. A typical whipantenna, 1 when properly tuned, has a voltage distribution extendingfrom zero at the base and to a voltage maximum at the high impedancepoint at its far or terminating, end. Somewhere between these twoextremes, an impedance value can be found to which a transmission linehaving a given characteristic impedance can be coupled. Prior artteaches that to properly tune such an antenna over a frequency range, ithas been necessary to adjust the length of the antenna to equal aquarter wavelength or multiple thereof and thus properly resonate at thetransmitted f rea quency and, in addition, to vary the couplingcharacteristic between the transmission line and the antenna tuning unitor antenna structure in order to provide a proper impedance matchbetween'the transmission line and the antenna. Prior art automatictuning devices have, genw erally made two measurements, one of thereactance. of the load and the other of the impedance of the load.Separate controlling elements were utilized to vary the length ofantenna or tuning elements to provide minimum reactance. and a secondcontrolling element was utilized to adjust the coupling characteristicto match the characteristic impedance of the input transmission line.Thus, these prior art devices wereprimarily dependent upon twomeasurements which controlled in some manner two adjustments. Obviously,these two adjustments could be done automatically by incorporatingseparate servo loops for each measurement in each control unit. I

. One of the objects of this invention, therefore, is to provide asimplified antenna tuning unit to resonate. an antenna at a givenfrequency within a wide frequencyband and to match the antenna to aninput transmission line by only a single adjustment.

Another object of this invention is to provide asimplified automaticantenna tuning unit in which'a single ads justment varies both thereactance of the antenna unit and the voltages at various points alongthe line are determined by the relative phase of these waves responsiveto the distance of the point from the load. This variation of voltageswith position along the line is called a standing wave and the ratio ofthe largest voltage, where the incident and reflected voltages addexactly in phase, to the smallest voltage, where the voltages are ofnearly opposing phase and thus cancel, is known as the standing waveratio or SWR.

When the load is matched, all the power coupled to the line is deliveredto the load and the reflected wave is zero. Thus, for matchedconditions, all points along the transmission line operate at the samevoltage and the ratio of maximum to minimum voltage, or SWR, is 1. Whenthe load isnot matched, power is reflected and-the reflected waves causeregions of high and low voltage along the line, producing a SWR greaterthan 1. Presence of the input impedance of the unit to provide anacceptable standing wave ratio on the transmission line. I

A further object of this invention is to provide a simplified automaticantenna tuning unit in which the coupling characteristic or impedance ofthe unit is varied as a function of the variation of the reactance ofthis system.

One feature of this invention is the use of tuning means in series withan antenna and to which mechanically coupled, is coupling means tocouple energy between a transmission line and the antenna. As the tuningmeans is adjusted, to vary the effective length of the antenna, due to amodified coupling characteristic integral withthe tuning unit, thecoupling means matches the antenna impedance to the transmissionline.

Another feature of this invention is the provision of an automaticantenna tuning unit including a helical tuning element connected to theantenna along which a movable short-to-ground is provided to vary theeffective length of transmissionline coupled to; the antenna and thusresonate the antenna system at the frequency of transmission. Power iscoupled into the helical line of the tuner from the transmitter by meansof a fixed coupling coil mounted on the movable short assembly insteadof the adjustable coupling usually provided in antenna tuning units.Various degrees of coupling are integrated into the tuner by means ofsuch devices as a non-uniform winding pitch on the helical tuningelement or a varying diameter in the helical tuning element or a varyingdiameter of tuning element core to provide a variable capacitivecharacteristic along the tuning coil length, or similar means to insurethat the coupling characteristic of the system maintains the inputstanding rave ratio on the transmission line below apredetermined eve 'The above-mentioned and other features and objects of this inventionwill become more apparent by reference to the following descriptiontaken in conjunction with the accompanying drawings, in which:

"1 is a schematic diagram partly in block form of a typical prior artautomatic antenna tuning unit;

Fig. 2 is a schematic circuit diagram partly in block form of anautomatic antenna tuning unit in accordance with .the principles of myinvention;

Fig.3 is a series of curves helpful in the explanation of the automaticantenna tuning unit shown in Fig. 2; and

Fig. 4 is a schematic view partly in cross-section of one embodiment ofan automatic antenna tuning unit in accordance with the principles of myinvention.

' Referring to Fig. 1 of the drawings, a typical prior art automatictuning unit for an antenna 1 is therein shown to include a helicaltuning element 2. As will be readily understoodby anyone skilled in theart, as the transmitter 3 is tuned over a range of frequencies, theantenna 1 is made to represent either an inductive or capacitive load.When the antenna represents an inductive load, the electrical length ismade greater than a quarter wavelength but less than a half wavelengthto make the tuner element capacitive and thus resonate the inductance ofthe antenna reactance, by varying the position of the movableshort-to-ground 4 along the helical tuning element 2. This places avoltage null, shown by Curve 5, at the grounded point and places a highimpedance at the radiator-terminal of the antenna and thus, it isapparent that somewhere between the shorted point 4 and the terminal ofthe antenna 1 is a coupling position to which the incoming transmissionline 6 may be matched. A coupling coil 7 is provided which is movedalong the length of helical'tuning element 2 until a proper impedancematch is obtained between the antenna tuning unit-and thetransmission-line 6. In order to automatically adjust the antenna tuningunit shown in Fig. 1, a reactance measuring bridge 8 is coupled to thetransmission line 6 by any well'known means and its output controls aservo amplifier 9 towhich a motor 10 is responsive. The motor 10 adjusts'the movable short along the length of the helical tuning element 2until the reactance bridge 8 indicates a resonant point. Sequentially orsimultaneously with the adjustment, an impedance measuring bridge 11 hasits output coupled through a servo amplifier 12 to control'motor 13tomove the coupling coil 7 along the helical tuning'element 2 to providea proper impedance match between the antenna tuning unit and thetransmission line 6. If the two bridge circuits are operating properly,the 'standing'wave ratio 'monitor'14 coupled to the transmission line 6between the transmitter 3 and the antenna tuningunit will;indicate aminimumstanding wave ratio. However, it should be noted that such asystem requires dual servo loops and two adjustments to be madeinterdependently of each other. Such a system is extremely complex anddifiicult to'maintain, as well as construct and fabricate.

Referring to Fig. 2, the simplified automatic antenna tuning unit inaccordance with the principles of my invention is therein shown,schematically, partly in block form. An automatic antenna tuning unit 15comprising cient to have the standing wave ratio monitor indicate a ahelical tuning element 16 in series with an antenna 17 is provided. Theoutput of a transmitter 18 is coupled along transmission line 19 andthrough a standing wave ratio monitor 20 to a coupling coil 21 whichinduces the energy coupled from the transmission line 19 into the tuningelement 16 for transmission by the antenna 17. A movable short 22 variesthe length of helical line coupled to the antenna 17 and thus causes theantenna element to resonate at the frequency of the output oftransmitter 18. Mounted on the movable short 22 is a fixed coupling coil21 so arranged that if a proper reactance is attained by the positioningof, short-to-ground 22, the coupling coil 21 will provide a reactancewhich is suffi- SWR of less than 4 to 1 which is considered satisfactoryand eflicient for almost all transmission conditions.

Referring to Fig. 3 of the drawings, a graph is therein-illustratedshowing the variation of antenna resistance with a variation infrequency. The curve of is illustrative of the resistance of an antennawithout a tuning coil and it shows that, for example, at 2 mc. theantenna may have a resistance of 1 ohm and reach a resistance of 500.ohms at anti-resonance at 9 mc. and then continue to have its resistancevalue oscillate approximately ohms. Curve 31 illustrates a similarresistance curve for an antenna in series with a tuning coil. The addedresistance of the tuning coil at 2 mc. causes the system to have ahigher resistance, such as 10 ohms, while at antiresonance at 9 mc. theadded resistance of the tuning coil causes the resistance of the antennasystem to be only 400 ohms. Thus it can be seen thatthe added resistanceof a tuning coil tends to flatten out the resistance curve of theantenna system. Assuming that it is desirable to match a 60 ohmtransmission line to the antenna system, the resistance of which isindicated by the dotted line resistance level 32, and knowing theposition of the resistance curve at 2 me. and at anti-resonance at 9mc., it is seen that the desired coupling factor may be designed as anintegral part of the tuner so that the standing wave ratio on thetransmission line will always be less than 4 to 1. This varying couplingfactor may be made integral with the tuning unit by various devices,such as varying the pitch of the coil or tapering the coil diameter, sothat at any point along the resistance curve the ratio of the antennasystem resistance to the transmission line characteristic is less than 4to 1. In any event the output of the SWR monitor is coupled through aminimum detector circuit 50 such as disclosed in copending applicationSerial No. 498,865 filed April 4, 1955 assigned to the same assignee asthis application. The output of the minimum detector is amplified anddrives motor 49 to stop the shorting means when a satisfactory SWR isattained. Due to .the integrated coupling factor when the short isstopped a satisfactory match is achieved.

Referring to Fig. 4 of the drawings, one embodiment of an antenna tuningunit according to the principles of my invention is shown. The antennatuning unit comprises an outer housing or shell 40 surrounding the maintuning coil 41. One end of the main tuning coil 41 is coupled through anadjustable capacitor42 to the antenna unit to be tuned 43. A couplingcoil 44 couples energy from a transmission line 45 into themain tuningcoil 41. The energy from transmission line .45 is coupled through anautotransformer 46 and across an adjustable capacitor 47. Energy issupplied to the transmission line 45 from a transmitter 51 coupledthrough a standing wave ratio monitor 52. The length of main tuning coil41 in series with the antenna 43 is varied by adjustment of the fingers48 along the axial length of the helical tuning coil. Fingers 48 shortout all turns of the tuning coil below their physical position sincethey make electrical contact with the housing 40 which isat groundpotential. It should be noted that a varying coupling characteristic isinserted into the main tuning unit coilby'varying the pitch of thewinding and the diameter of the coil. It is of course obvious that othermethods of adjusting the coupling characteristic of this system may beused, such as a conductive core of varying diameter coaxial with themain tuning unit or means to physically rotate coupling coil 44 relativeto the axis of the main tuning coil 41.

In operation, the shorting fingers 48 of the antenna tuning coil aremoved to the top of the helix 41 by means of motor 49 and caused tostart their descent. The shorting fingers are started at the top of theantenna tuning coil because for maximum efiiciency it is desirable tohave the minimum length in the antenna system. As a resonant conditionis reached, i. e.,a condition in which the length of antenna 43 plus thelength of tuning coil 41 is equal to a quarter wavelength at theiroperating frequency, the standing wave ratio monitor 43 detects a SWR ofless than 4 to 1 and thus couples a signal to motor 49 enabling themotor to cease moving the shorting fingers 48 and thus resonate theantenna system. If

desired, a minimum sensing circuit may be utilized to anticipate theposition at which the shorting fingers should be stopped to obtain thestanding wave ratio desired. The coupling coil 44 is mounted in fixedrelation to the shorting fingers at such a point that when the shortingfingers resonate the antenna system for the predetermined operatingfrequency, the coupling characteristic between the coupling coil 44 andthe antenna tuning unit 41 provides a match for the transmission line45. If a helix of constant diameter will not provide such a satisfactorymatch, then the diameter of the helix 41 may be altered to provide adifferent coupling characteristic for the particular frequency or thepitch of the winding may be altered so that when the fingers 48 arepositioned in this location, the coupling characteristic will besatisfactory to provide a good standing wave ratio.

In order to extend the range of frequencies through which this antennatuning system may be utilized, an additional tuning capacitor 42 may beadded in series with 43 as is well known to those skilled in the art. Ifdesired, at the operating frequency a variable capacitor 47 may becoupled across the coil 44 in order to resonate the coupling coil. Inaddition, the range of the antenna tuning unit may be extended byproviding an autotransformer having a turns ratio in the order of 3 to 1and thus extend the range over which a match can be attained.

It will be readily understood that since the coupling coil 44 is infixed relation to 48 but a varying coupling characteristic is madeintegral in the design of the antenna tuning unit 41 that the couplingresistance or impedance will vary with the position of the shortingfingers. It is also obvious that by varying the diameter of a coremember which could be added to the helical tuning unit, capacitiveloading is obtained which could function as a top loading when the shortcircuit fingers 48 are in a position below the largest diameter of thecore, which would act as a low impedance region. It is also obvious thatvarying the coupling characteristic varies the resonance frequency ofthis system and therefore enables the coupling coils to match theimpedance of the transmission line.

While I have described above the principles of my invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of my invention, as set forth in the objects thereof and inthe accompanying claims.

I claim:

1. An antenna tuning and coupling unit for coupling energy over a wideband of frequencies from a transmission line to an antenna comprising,an inductance coil in series with said antenna, adjustable shortingmeans to short circuit to ground a variable portion of said coil toadjust the effective electrical length of said antenna and coil, meansmechanically coupled to said adjustable shorting means for movementtherewith to couple energy between said transmission line and said coil,means to modify the coupling characteristic between said coil and saidcoupling means in accordance with the position of said coupling meansrelative to said coil for each position of said adjustable shortingmeans.

2. An antenna tuning unit for coupling energy over a wide band offrequencies from a transmission line to an antenna comprising, a helicaltuning coil in series with said antenna, adjustable shorting means toshort circuit to ground a variable portion of said tuning coil to adjustthe effective electrical length of said antenna and tuning coil, meanscarried by said adjustable shorting means to couple energy between saidtransmission line and said tuning coil, means integral with said tuningcoil to vary in a non-uniform manner along the axial length of saidhelical tuning coil the coupling characteristic between said tuning coiland said coupling means.

3. An antenna tuning unit for coupling energy over a wide band offrequencies from a transmission line to an antenna comprising, a helicaltuning coil in series with said antenna, adjustable shorting means toshort circuit to ground a variable portion of said tuning coil to adjustthe effective electrical length of said antenna and tuning coil, meanscarried by said adjustable shorting means to couple energy between saidtransmission line and said tuning coil, said helical tuning coil havinga non-uniform winding pitch whereby the coupling characteristic betweensaid tuning coil and said coupling means is altered responsive to theposition of said adjustable shorting means.

4. An antenna tuning unit for coupling energy over a wide band offrequencies from a transmission line to an antenna comprising, a helicaltuning coil in series with said antenna, adjustable shorting means toshort circuit to ground a variable portion of said tuning coil to adjustthe effective electrical length of said antenna and tuning coil, meanscarried by said adjustable shorting means to couple energy between saidtransmission line and said tuning coil, said helical tuning coil havinga non-uniform diameter whereby the coupling characteristic between saidtuning coil and said coupling means is altered responsive to theposition of said adjustable shorting means.

5. An antenna tuning unit for coupling energy over a wide band offrequencies from a transmission line to an antenna comprising, a helicaltuning coil in series with said antenna, adjustable shorting means toshort circuit to ground a variable portion of said tuning coil to adjustthe effective electrical length of said antenna and tuning coil, acoupling coil carried by said adjustable shorting means in fixedrelation thereto to couple energy between said transmission line andsaid tuning coil, means integral with said tuning coil to modify thecoupling characteristic between said tuning and coupling coils for eachposition of said adjustable shorting means.

6. An antenna tuning unit for coupling energy over a wide band offrequencies from a transmission line to an antenna comprising, a coaxialstructure including an outer conductive casing member, a helical tuningcoil in series with said antenna and disposed in coaxial relation tosaid casing member, adjustable shorting means to couple a point on saidhelical coil to said casing member, a coupling coil carried by saidadjustable shorting means coaxial to said tuning coil to couple energybetween said transmission line and said tuning coil, means integral withsaid tuning coil varying in a non-uniform manner along said tuning coilsaxial length to modify the coupling characteristic between said tuningcoil and said coupling coil in accordance with the position of saidadjustable shorting means.

References Cited in the file of this patent UNITED STATES PATENTS2,498,078 Harrison Feb. 21, 1950 2,515,436 Babin July 18, 1950 2,657,362Epperson Oct. 27, 1953 2,745,067 True et a1. May 8, 1956

